The perovskite-based ceramic oxides are well known due to their excellent oxidation resistances, chemical stabilities, structural, electrical, magnetic, and mechanical properties. Owing to these properties oxide-based ceramics are widely used in numerous areas.^[1–3] RFe_1−xB_xO₃ orthoferrites (where R denotes a rare earth ion and B a transition metal ion such as Ni, Co, Mn, Cr) are one of the most studied classes of perovskites. Doping at B-sites affects the physical properties with change in their ionic radii, oxidation states and spin states which play a key role in determining the structural, electronic and magnetic properties.^[3–6]

The structure of NdFeO₃ is orthorombically distorted derivative of cubic perovskites with Pbnm symmetry. The crystallographic unit cell of NdFe_1−xNi_xFeO₃ can be visualized as three-dimensional (3D) tilted, slightly deformed and corner sharing FeO₆ octahedron while the A-site cations (Nd³⁺) are dodecahedrally coordinated by 12 O²⁻ ions and fit in the cavities comprised of eight FeO₆/NiO₆ octahedra. The Neel temperature of NdFeO₃ is 760 K which decreases with the increase in nickel content. Each Fe ion is coupled by superexchange interaction with six nearest Fe ions, resulting in antiferromagnetic ordering while weak ferromagnetism is observed due to canting of spins.^[5–9]

Strong interactions among localized moments, magnetic ordering, itinerant electrons, and spin canting, normally result in the interesting phenomenon in these materials. Owing to these factors, the ABO₃ type perovskite materials exhibit huge variations in properties; which are observed due to the changes in oxidation and spin states, local structural, and chemical factors, difference in band gap, octahedral tilting, cations displacements, colossal polarizeability (with electronic, ionic, and dipolar polarization mechanisms) and relaxer like properties. These perovskites are widely studied both theoretically and experimentally due to very broad range of electrical properties from insulators to superconductors, metal to insulator transitions, magnetic properties, magneto-resistive properties, electrochemical and catalytic properties. Owing to these properties transition metal oxides have become important materials for several high-tech applications such as solid oxide fuels cells, gas sensors, catalysis, spin valves, magnetic field sensors, magnetic sensors, electro-magnets, spintronic devices, oxygen permeable membranes, environmental monitoring applications, random access memory devices based on capacitive components, miniaturization of device sizes, multilayer ceramic capacitors, dielectric substrates, thermoelectric power conversion microwave components for communication systems, etc.^[10–20]

When Ni is doped at the Fe sites in orthoferrites, a transition from insulating to metallic state is observed. The difference in ionic radius between Fe³⁺ and Ni³⁺ produces structural distortions. According to the rigid band model, the Fe³⁺ ions have more localized states as compared with the Ni³⁺ ions, thus the nickel doping in orthoferrites increases the concentration and delocalization of charge carriers.^[5,21,22] The RNiO₃-type nickelates are paramagnetic metallic oxides due to the presence of Ni³⁺ in low spin state with its e_g electrons having no localized moments. The t_2g band of these systems is completely filled and conduction band is created by the hybridization of the Ni³⁺ e_g orbitals and O²⁻ 2p orbitals.^[23] In NdFeO₃ the Fe ions are in high spin state. The Ni doping in this system can increase oxidation state of the iron ions only if the nickel ions are present in mixed oxidation states (+2 and +3) and oxygen stoichiometry of the system is also maintained which may result in an increase in hole state. Idrees et al. have observed increase in hole states and presence of Fe⁴⁺ with the increase in Ni concentration in LaFe_1−xNi_xO₃.^[24,25] Makhdoomi et al.^[26] have reported the transition of Fe³⁺ ions from high spin to low spin in PrFe_1−xNi_xO₃ orthoferrite system with the increase in Ni concentration. Fe, Co and Ni are neighbors in the periodic table and NdFeO₃, NdCoO₃, and NdNiO₃ crystallize in the same crystal structure; however, NdFeO₃ is an insulator, NdCoO₃ is a semiconductor and NdNiO₃ shows a metallic behavior.^[7] The metallic or insulating behaviors of these materials dependon the extent of distortion from ideal cubic perovskite structure and on B–O–B bond angle.^[5] Rao et al.^[27] studied the LaNi_1−xFe_xO₃ system and found that the itinerancy raises with the increase in the nickel concentration. They also observed that the increase in covalent character of B–O bond results in itinerant behavior of perovskite oxide, with B-site cation in the low spin state. Furthermore, the charge carriers of ABO₃ oxide having higher ionic character of B–O bond will be localized and in this case the B-site cations will be in the high spin state.

In NdFe_1−xNi_xO₃ system both Fe and Ni are magnetic 3d transitions metals, and show magnetic ordering below T_N which is ∼ 700 K for NdFeO₃ and ∼ 200 K for NdNiO₃. The exchange interaction between B-site ion and Nd ions can be divided into isotropic and anisotropic components. The isotropic exchange is canceled by compensating for contributions from anti-parallel moments. However, the anisotropic exchange is not completely compensated for, which results in the appearance of internal field H_Nd–B and polarizes the Nd sub-lattice at lower temperatures both in NdFeO₃ and NdNiO₃. Magnetic vacancies are introduced into the Fe sub-lattices when iron ions are substituted by non-magnetic ions. This results in the dilution of the Fe sub-lattices which stimulates the lowering of Neel temperature till quenching of magnetic ordering.^[28]

Tin oxide and zinc oxide are widely studied materials as gas sensors but these materials have poor stabilities and selectivities which restricted their usefulness as gas sensing materials. Doped and undoped perovskite oxides are other types of materials which are extensively studied as gas sensors due to superior selectivity. Zhang et al.^[29] have studied the CO gas sensing properties of NdFe_1−xCo_xO₃ system and observed that NdFeO₃ has very good response as CO gas sensor.

In this work NdFe_1−xNi_xO₃ orthoferrites are synthesized by solid state reaction method and characterized by XRD, pycnometer, SEM and Mössbauer spectroscopy.

Polycrystalline NdFe_1−xNi_xO₃, (where x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, and 0.7), samples were synthesized in air by a conventional solid state reaction method, using high purity (99.99%) Nd₂O₃, NiO and Fe₂O₃ obtained from Sigma Aldrich. To obtain a homogenous mixture, stoichiometric quantities of all the chemicals were mixed in highpurity acetone followed by grinding. Appropriate measures were taken to ensure that starting materials are moisture free, by preheating at 100 °C for 2 h before weighing. The mixtures of oxides were heated at 1000 °C, 1100 °C, and 1200 °C for 16 h at each temperature with intermediate grinding. After this heat treatment, circular pellets of 10-mm diameter and ∼ 1.5-mm thicknesses were fabricated from the ground powder under a pressure of 3 tons/inch² (1 ton = 1.01605 t, 1 inch = 2.54 cm). These pellets were finally sintered at 1300 °C for 16 h. Heating rate was maintained at 5 °C/min during sintering at each temperature, while all samples were cooled in closed off furnace. The phase purity of each sintered pellet was confirmed by x-ray diffraction (XRD) using Cu K_α radiation (1.5418 Å). The room temperature XRD data were collected for 20° ≤ 2θ ≤ 70° with 0.02° scanning step size and 3 s per step counting time. In order to calculate the lattice parameters, least square fitting technique was used. Densities of these sintered pallets were measured using pycnometer. SEM analyses of all these samples were performed using SEM LEO 440i. For SEM analysis each sintered pallet was fractured perpendicularly to the circular plane and analyses of these fractured samples were carried out. Fe⁵⁷ Mössbauer spectra were taken at room temperature in transmission mode using the Co⁵⁷ in Rh matrix as a source of γ-rays. For the calibration of Mössbauer spectrometer, a thin α -Fe foil was used. MOS-90 computer program was used to analyze the Mössbauer data; where analysis was performed on the assumption that all peaks are Lorentzian in shape.

Figure 1 shows the XRD patterns of freshly synthesized samples of NdFe_1−xNi_xO₃, (where x = 0.1–0.7), collected at room temperature. All peaks are successfully indexed on the basis of PCPDF card Nos. 82-2421, 82-2422, 82-2158, and 81-2245.^[5,6,30] The XRD results show that single phase orthorhombic perovskite structure having space group Pbnm has been synthesized. However in 50% Ni-doped sample two very small peaks due to a minor tertragonal Nd₄Ni₃O₈ phase are observed. The lattice parameters, a, b, and c, calculated from XRD data are shown in Table 1 which are consistent with those earlier reported.^[4,5,31,32] With the increase in the nickel concentration, XRD peaks shift towards higher 2θ indicating that the cell volume decreases as the Ni concentration is increased. This decrease in unit cell volume might be due to the difference in ionic radius between Ni³⁺ (0.60 Å) and Fe³⁺ (0.645 Å) in high spin states.^[33]

Fig. 1.

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	Fig. 1. XRD patterns of (curve a) NdFe0.9Ni0.1O3, (curve b) NdFe0.8Ni0.2O3, (curve c) NdFe0.7Ni0.3O3, (curve d) NdFe0.6Ni0.4O3, (curve e) NdFe0.5Ni0.5O3, (curve f) NdFe0.4Ni0.6O3, and (curve g) NdFe0.3Ni0.7O3 collected at room temperature. Symbol * indicates peaks due to a minor tetragonal Nd4Ni3O8 phase.

Table 1.

Table 1.

Table 1. Phase symmetry, cell parameters a, b, and c, and cell volumes of NdFe1−xNixO3. . Sample Phase symmetry a/Å b/Å c/Å Cell volume/Å3 NdFe0.9Ni0.1O3 orthorhombic 5.4540 5.5805 7.7649 236.333 NdFe0.8Ni0.2O3 orthorhombic 5.4448 5.5708 7.7524 235.145 NdFe0.7Ni0.3O3 orthorhombic 5.4433 5.5629 7.7359 234.247 NdFe0.6Ni0.4O3 orthorhombic 5.4404 5.5447 7.7351 233.332 NdFe0.5Ni0.5O3 orthorhombic 5.4400 5.5381 7.7219 232.640 NdFe0.4Ni0.6O3 orthorhombic 5.4396 5.5412 7.7083 232.343 NdFe0.3Ni0.7O3 orthorhombic 5.4393 5.5368 7.6717 231.043

Table 1. Phase symmetry, cell parameters a, b, and c, and cell volumes of NdFe1−xNixO3. .

In Table 2, the values of Goldsmith tolerance factor are given, which show an increasing trend with the increase in nickel concentration. Goldsmith tolerance factor (t) has been calculated by using the following relation:

Table 2.

Table 2.

Table 2. Calculations of Goldsmith tolerance factor, orthorhombic distortion parameter, XRD density, bulk density, and calculated porosity of NdFe1−xNixO3 samples. . Sample Goldsmith tolerance factor (t) Orthorhombic distortion parameter (D) XRD density dth/g·cm−3 Bulk density d/g·cm−3 Calculated porosity P/% NdFe0.9Ni0.1O3 0.92525 0.8714 6.97946 6.84098 1.9841 NdFe0.8Ni0.2O3 0.92729 0.8673 7.02288 6.91294 1.5654 NdFe0.7Ni0.3O3 0.92935 0.8579 7.05776 6.95674 1.4313 NdFe0.6Ni0.4O3 0.93141 0.7260 7.09355 7.00733 1.2154 NdFe0.5Ni0.5O3 0.93348 0.7127 7.12278 7.03495 1.2331 NdFe0.4Ni0.6O3 0.93557 0.7787 7.14003 7.04467 1.3356 NdFe0.3Ni0.7O3 0.93766 0.8601 7.18839 7.07634 1.5587

Table 2. Calculations of Goldsmith tolerance factor, orthorhombic distortion parameter, XRD density, bulk density, and calculated porosity of NdFe1−xNixO3 samples. .

For the calculation of tolerance factors the ionic sizes are cited from Ref. [33] by Shannon. It is evident from these results that the values of Goldsmith tolerance factor (t) increase slightly with the increase in Ni.

The orthorhombic distortion parameter (D), which is a measure of deviation of unit cell from ideal cubic structure and is represented by the following formula:^[34]

In this formula, α₁ = a, α₂ = b, , and

Fig. 2.

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	Fig. 2. SEM micrographs (at 3000× magnification) taken from the fractured surfaces of the sintered pellets of (a) NdFe0.9Ni0.1O3, (b) NdFe0.8Ni0.2O3, (c) NdFe0.7Ni0.3O3, (d) NdFe0.6Ni0.4O3, (e) NdFe0.5Ni0.5O3, (f) NdFe0.4Ni0.6O3, and (g) NdFe0.3Ni0.7O3.

The density of the material is an important parameter; the bulk density must be above ninety percent of the theoretical density for the determination of electrical and thermal properties.^[35] The bulk density of each of NdFe₁−xNi_xO₃ samples is measured using pycnometer while the theoretical density is calculated from the following equation:

The results of theoretical density (d_th), bulk density (d) and calculated porosity (P) are also shown in Table 2. From these results it is observed that in all samples porosity values are less than 2 percent and bulk density is above 98 percent as compared with the theoretical density which is advantageous for measuring the electrical and thermal properties.

In order to investigate the effects of nickel doping on the particle size, porosity, grain connectivity and grain boundary, we have carried out scanning electron microscopy (SEM). The SEM micrographs of the NdFe_1−xNi_xO₃ pellets (fractured face) sintered at 1300 °C are shown in Figs. 2(a)–2(g) at 3000× magnification. The SEM analyses are performed at room temperature; from these micrographs the grain connectivities, grain boundary areas, grain sizes, and grain shapes are clearly visible. The lower doping samples have more regular grain shapes and with the increase in Ni content the grain connectivity and grain size gradually increase. The SEM results infer that grains intermixing increase with the increase in Ni concentration. Some grains, especially in lower doping samples, have well defined geometrical shapes and sharp grain boundaries; with the increase in nickel content there is a gradual reduction in grain boundary area. The observed pore size varies from area to area and sample to sample but mostly their sizes are all less than 1 μm. The pores are produced mainly due to the effect of sintering temperature, applied pressure and pressing time for making palates while the disorientation and mismatch of grains and grain boundaries can also play an important role.

The effects of Ni doping on the local environment of Fe sites in NdFe_1−xNi_xO₃ (0.1 ≤ x ≤ 0.7) are investigated by Mössbauer spectroscopy to explore the roles of Ni doping in the oxidation state of Fe, octahedral distortions and the change in the strength of magnetic ordering. The Mössbauer spectra of NdFe_1−xNi_xO₃ (0.1 ≤ x ≤ 0.7) samples are shown in Fig. 3 and their fitted parameters are summarized in Table 3. The Mössbauer spectrum for 10% Ni is very well fitted with a magnetic sextet and a small paramagnetic singlet [Fig. 3(a)]. In an earlier study we have reported the Mössbauer spectrum of NdFeO₃ in which only a magnetic sextet was observed.^[35,36] This indicates that the appearance of a small singlet is due to paramagnetic phase formed due to Ni doping. The internal magnetic, H_eff also decreases with Ni doping indicating the paramagnetic behavior of Ni in this system. From the isomer shift values it is inferred that oxidation state of iron is Fe³⁺ in the whole system. The quadrupole splitting is very sensitive parameter to probe the local environment of Fe and its value close to zero represents no distortion in Fe octahedron in this system. The line broadening can be related to the hyperfine field distribution originating from the non-equivalent Fe sites in the crystal symmetry.

Fig. 3.

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	Fig. 3. The room temperature Mössbauer spectra of (a) NdFe0.9Ni0.1O3, (b) NdFe0.8Ni0.2O3, (c) NdFe0.7Ni0.3O3, (d) NdFe0.6Ni0.4O3, (e) NdFe0.5Ni0.5O3, (f) NdFe0.4Ni0.6O3, and (g) NdFe0.3Ni0.7O3.

The Mösbasuer spectra of NdFe_1−xNi_xO₃ (0.2 ≤ x ≤ 0.5) are shown in Figs. 3(b)–3(e) and the fitted parameters of these samples are given in Table 3. In these samples the local distortions and strains caused by the A-site cations keep almost similar and changes are mainly observed due to B-site cations. The spectra of these four samples are fitted by two sextets and one singlet. The appearance of more than one sextet is ascribed to dissimilar distributions of nickel ions around iron ions. The decrease in internal magnetic field (H_eff) clearly shows that the magnetic ordering decreases with the increase in nickel concentration. The quadrupole splitting values increase gradually with Ni content increasing up to 40% indicating that there is an increase in the concentration of Fe ions which are slightly shifted from their octahedral sites. Thus the octahedral distortions in the local structure increase up to 40%. However, for 50% Ni doped sample a decrease in quadrupole splitting is observed which may be because more symmetric arrangements, owing to the equal number of Fe and Ni atoms at B-sites, form BO₆ octahedra. With the increase in Ni concentration the line width also increases, indicating an increase in sagging and distribution of hyperfine parameters. The line broadening observed in these samples indicates the gradual collapse of magnetically ordered state into spin disordered state. The decrease in average internal magnetic field with the increase of Ni-concentration, as deduced from the Mössbauer results, can be seen in Fig. 4.

Table 3.

Table 3.

Table 3. Values of magnetic hyperfine field (HB), isomer shift (IS), quadrupole splitting (Δ), line width (Γ), and relative area (%) associated with each spectral feature in 57Fe Mössbauer spectra of NdFe1−xNixO3. The unit 1 Oe = 79.5775 A·m−1. . Sample Heff/kOe IS/mm·s−1 Δ/mm·s−1 Γ/mm·s−1 Relative area/% NdFe0.9Ni0.1O3 sextet A 513 0.38 −0.010 0.44 99 singlet — 0.41 — 1.17 1 NdFe0.8Ni0.2O3 sextet A 495 0.39 −0.026 0.41 30 sextet B 470 0.37 −0.027 0.80 68 singlet — 0.34 — 0.65 2 NdFe0.7Ni0.3O3 sextet A 441 0.36 −0.041 0.66 34 sextet B 399 0.36 −0.036 1.34 63 singlet — 0.29 — 0.68 3 NdFe0.6Ni0.4O3 sextet A 414 0.36 −0.058 0.86 37 sextet B 362 0.28 −0.151 1.61 47 singlet — 0.32 — 0.89 16 NdFe0.5Ni0.5O3 sextet A 411 0.35 0.041 1.82 16 sextet B 355 0.35 −0.019; 5.88 77 singlet — 0.30 — 1.32 7 NdFe0.6Ni0.4O3 singlet — 0.36 — 1.07 100 NdFe0.7Ni0.3O3 singlet — 0.37 — 0.99 100

Table 3. Values of magnetic hyperfine field (HB), isomer shift (IS), quadrupole splitting (Δ), line width (Γ), and relative area (%) associated with each spectral feature in 57Fe Mössbauer spectra of NdFe1−xNixO3. The unit 1 Oe = 79.5775 A·m−1. .

Fig. 4.

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	Fig. 4. Variation of average internal magnetic field with Ni concentration in NdF1−xNixO3.

The Mösbasuer spectra of NdFe_1−xNi_xO₃ (0.6 ≤ x ≤ 0.7) are shown in Figs. 3(f) and 3(g) and the fitted parameters of these samples are also given in Table 3. The Mössbauer spectra of both samples are fitted with a singlet only indicating that magnetic ordering is completely lost from the samples doped with 60% and higher Ni.

As observed by the Mössbauer spectra and broadening or sagging of the magnetically split lines, a progressive collapse of magnetic ordering is observed with the increase in Ni concentration of NdFe_1−xNi_xO₃ (0.1 ≤ x ≤ 0.7). It is interesting to note that the collapse of magnetic ordering is also observed in other Ni-doped LaFeO₃, PrFeO₃, and NdFeO₃ orthoferrite systems.^[4,37–39]

Makhdoomi et al.^[38] observed that in PrFe_1−xNi_xO₃ system when octahedral distortions around Fe³⁺ increase, it results in the increase in crystal field splitting energy which becomes higher than average spin pairing energy of electrons. Consequently, due to the spin crossover the high spin configuration of Fe³⁺ (with 5 unpaired spins) is converted into low spin (with only one unpaired spin) one. But the observed magnetic field values for up to 50% Ni samples are higher than the expected field due to low spin of Fe³⁺ with 5 t_2g and zero e_g electrons in the valence shell. Thus up to 50% Ni concentration samples, Fe is not converted into low spin configuration while in higher doped samples it may be the case.

In the case of LaFe_1−xNi_xO₃ system, Marzec^[39] explained the transition from a magnetically ordered state to a spin disordered state by a spin–spin relaxation phenomenon which becomes faster than the characteristic Mössbauer experiment time window with Ni doping. The increase in spin–spin relaxation frequency with temperature can account for the temperature-dependent sagging of the sextet and collapse of the magnetically order state above Neel temperature into orthoferrites.^[37–40] With the increase in nickel concentration, a progressive increase in itinerant behavior is observed and results in less localized electron spins. Consequently, the spin relaxation becomes easier which results in the progressive collapse of the magnetically ordered state into spin disordered state. Asai and Sekizawa^[41] also established that in these materials progressive sagging of sextet increases with the decrease in characteristic relaxation time constant.

The strength of the super-exchange interaction in orthoferrites depends upon the nature of B-site cation and the B–O–B bond angle. Thus the collapse of magnetic ordering, sagging behavior and decrease in Neel temperature can also be correlated with the weakening of super-exchange interactions. As a result of doping, the Fe³⁺–O²⁻–Fe³⁺ interactions in NdFeO₃ become Fe³⁺–O²⁻–Fe³⁺, Fe³⁺–O²⁷⁻–Ni³⁺, and Ni³⁺–O²⁻–Ni³⁺ interactions in NdFe_{1 −x}Ni_xO₃ (0.1 ≤ x ≤ 0.7). The super-exchange angle also changes which leads to a decrease in super-exchange interactions with the increase in Ni content. It is inferred that all above factors contribute to the collapse of magnetic ordering with the increase of Ni concentration in NdFe_1−xNi_xO₃.

In the present study polycrystalline NdFe_1−xNi_xO₃ (0.1 ≤ x ≤ 0.7) orthoferrites are synthesized by a solid state reaction method. From the XRD results it is found that the orthorhombic structure is retained in each of all samples; however, by increasing the Ni concentration, tolerance factor increases while cell volume decreases. The orthorhombic distortion is found to decrease with Ni concentration increasing up to 50%, while above 50% Ni doping it starts to increase. The calculated porosity value in these systems are all below 2%. From SEM results it is found that when the concentration of nickel is low, the grains have more regular shapes and with the increase in Ni concentration grain connectivity grain size gradually increases. From these results it is also inferred that the intermixing grains increase with the increase in Ni concentration. The pores size differs from area to area and sample to sample but mostly their sizes are all less than 1 μm. Mössbauer spectroscopic results indicate that the oxidation state Fe ion is Fe⁺³ for each of all samples. The increase in distribution of hyperfine parameters and the decrease in magnetic ordering with the increase in nickel concentration are also observed. The major factors behind the collapse of magnetic ordering in this orthoferrite system may be the weakening of superexchange interactions, the transition from high spin to low spin and the increase in spin–spin relaxation frequency due to the increase in the delocalization of charge carriers.